Polymorphism in Ceapaea nemoralis in Monks Riseborough
Introduction
We will be studying the polymorphism in the snail species Ceapaea nemoralis. This species exhibits very obvious, tangible polymorphism in their shell colours and bandings. The populations can be easily monitored as individuals do not travel over long distances because of their slow movement. We attempt to observe patterns in changes in allele frequency of sampled populations. Furthermore the evolutionary principles acting on and affecting the polymorphism in snails can be applied to human populations. Here polymorphic traits are perhaps harder to distinguish and therefore harder to apply to either of the principles.
The sampling of our experiment aims to show evidence of increased impact of gene flow in boundary zones compared to that seen in areas which exhibit more consistent habitat. Populations living in or in close proximity to habitat boundaries are expected to exhibit a higher degree of polymorphism due to gene flow from populations on which visual and climatic selection is acting. Visual selection is imposed by predators choosing the more visually exposed individual. Climatic selection can act on the phenotype of the individual, as for example dark snails retain heat better than lighter coloured snails. This can affect the frequency at which certain phenotypes are found at particular altitudes.
We will be taking samples at a number of different locations at similar altitudes, which will each consist of an intermediate zone and two types of habitat on either side considered to be more isolated making migration from different types of habitats less likely. Selection and genetic drift will therefore have more impact than gene flow at these locations. We examine variations in allele frequency in terms of both banding and colouration between the different sites.
The intermediate zone is expected to show a higher rate of polymorphism than the habitats on either side, these areas are expected to show morphs with a phenotype favoured by visual and climatic selection.
A wide variety of shell colours and bandings in intermediate zones would support the theory that gene flow does in fact affect the allele frequencies, maintaining polymorphism in the population. In comparison, phenotypically uniform populations in habitats on either side and some distance from the intermediate zone would support the hypothesis that gene flow has smaller impact on allele frequencies in areas exhibiting more consistent habitat.
If however allele frequencies in samples from similar habitats differ markedly, this might suggest that genetic drift has had an impact on the population or that the selection pressures differ between locations. This should be taken into account when reviewing results. This experiment assumes that selection works against maintaining polymorphism. A confounding factor could be the apostatic selection presented by the predatory birds.
Two degrees of freedom and 0.05 critical value (5.99)
Location 1
Colour data:
Location 1
Yellow
Pink+ Brown
Total
Grassland
7
3
10
Intermediate
8
2
10
Shrub
6
4
10
Total
21
9
30
Expected
7
3
Chi square: 0.94 - null hypothesis accepted
Banding data:
Location 1
0-3 band
4-5 bands
Total
Grassland
6
4
10
Intermediate
5
5
10
Shrub
3
7
10
Total
14
16
30
Expected
4.67
5.33
Chi square: 1.87 - null hypothesis accepted
No significant difference in morph frequencies for colour or banding. We see the expected trend in the grassy area with a higher frequency of yellow and 4-5 banded shells. The intermediate area showed a variety of polymorphism, slightly skewed towards yellow shells. In the shrubby area we expect a high ratio of pink/brown coloured shells, however we found more yellows. This area was relatively grassy, so selection would favour yellow, banded as they would be better camouflaged. Gene flow from the grassy area could be affecting the shrub and intermediate zone.
Location 2
Colour data:
Location 2
Yellow
Pink+Brown
Total
Grassland
8
2
10
Intermediate
7
3
10
Shrub
1
9
10
Total
16
14
30
Expected
5.33
4.67
Chi square: 11.53 - null hypothesis rejected
Banding data:
Location 2
0-3 band
4-5 bands
Total
Grassland
8
2
10
Intermediate
1
9
10
Shrub
9
1
10
Total
18
12
30
Expected
6
4
Chi square: 15.84 - null hypothesis rejected
Significant differences in frequency for both colour and banding, with a high number of yellow in both grassy and intermediate zones, with more variation in the intermediate zone than expected. The trend was shown in the shrub area, which was woodier than location one, pink/brown unbanded shells were most frequent. This obeys the expected trend; variation was found to be highest in the intermediate zone, suggesting that the ‘confined’ zones would be subject to selection favouring yellow in grassland and pink/brown in shrub area.
Location 3
Colour data:
Location 3
Yellow
Pink+Brown
Total
Grassland
10
0
10
Intermediate
4
6
10
Shrub
4
6
10
Total
18
12
30
Expected
6
4
Chi square: 10.01 - null hypothesis rejected
Location 3
0-3 band
4-5 bands
Total
Grassland
0
10
10
Intermediate
8
2
10
Shrub
8
2
10
Total
16
14
30
Expected
5.33
4.67
Chi square: 17.15 - null hypothesis rejected
Significant differences in morph frequency for both colour and banding as expected, the grassy area exhibited a high frequency of yellow, banded shells. The intermediate area showed a variety of polymorphism, slightly skewed towards yellow and banded which indicated gene flow from the grassland or selection favouring this phenotype. The shrub area showed the expected higher frequencies of pink/brown, unbanded.
Location 4
Colour data:
Location 4
Yellow
Pink+Brown
Total
Total
Grassland
5
5
10
10
Intermediate
5
5
10
10
Shrub
3
7
10
10
Total
13
17
30
30
Expected
4.33
5.67
Chi square: 1.087 - null hypothesis accepted
Banding data:
Location 4
0-3 band
4-5 bands
Total
Grassland
4
6
10
Intermediate
6
4
10
Shrub
3
7
10
Total
13
17
30
Expected
4.33
5.67
Chi square: 1.83 - null hypothesis rejected
No significant morph frequency for colour and banding. As expected, in grassland yellow, banded were most frequent, however this trend was not as pronounced as other locations. The intermediate area showed variety, though perhaps because the grassland and shrub locations either side had more yellow, pink/brown and banded this was reflected in our findings. The shrub area did not exhibit expected frequencies as there was a majority of banded shells but overall pink/brown shells were more frequent.
Discussion:
The discrepancies we found in certain locations could have arisen from sampling errors or bias which might have caused anomalous results. For example our sample size was too small with only 10 individuals in each. In location 3 the sample could have been larger, but in the remaining areas we struggled to find even 10 individuals however we aimed to keep the sample size consistent across the locations we sampled. The grassland and shrub areas that we sampled could have been more confined; in reality there was a more gradual change in habitat which could account for the data being skewed towards yellow and banded shells.
The fact that so few pink/brown shells were found could be due to them being harder to see. In the shrub area where these would be expected we were forced to search underneath vegetation and leaf litter, which may have caused us to overlook the brown shells. Yellow and pink were much easier to spot which would produce a bias. Location 1 might differ because this was our first attempt and from here on we were more aware of what we were looking for thus searching became more efficient.
Overall our results support our hypothesis that polymorphism is more pronounced in the intermediate zones which is likely to be due to the actions of gene flow from populations in confined areas. The confined populations are expected to exhibit less polymorphism as they are subject to both selection and genetic drift. Considering previous results (Jones et al 1977) it would be expected that in a shrub area you would find more brown unbanded snails whereas in a grassland area there would be more yellow banded snails. This is because predators are prone to select their prey on the basis of appearance so camouflage within a habitat will provide a selective advantage. However in location 1 the distribution of shell colours was skewed towards yellow which could be a result of genetic drift. Location 4 also differed from our expectations in that the shrub area did not seem to be as confined as it could have been. The results suggest that gene flow from outside populations could have skewed our results. The nature of these evolutionary processes means that the polymorphism seen in the snail populations can not be attributed to just one factor but a combination of effects will produce frequencies within a population.
Contributing members (all):
Henry Pyett
Richard Morrow
Laura Shakespeare
Kathrine Nielsen (group secretary)
Introduction
We will be studying the polymorphism in the snail species Ceapaea nemoralis. This species exhibits very obvious, tangible polymorphism in their shell colours and bandings. The populations can be easily monitored as individuals do not travel over long distances because of their slow movement. We attempt to observe patterns in changes in allele frequency of sampled populations. Furthermore the evolutionary principles acting on and affecting the polymorphism in snails can be applied to human populations. Here polymorphic traits are perhaps harder to distinguish and therefore harder to apply to either of the principles.
The sampling of our experiment aims to show evidence of increased impact of gene flow in boundary zones compared to that seen in areas which exhibit more consistent habitat. Populations living in or in close proximity to habitat boundaries are expected to exhibit a higher degree of polymorphism due to gene flow from populations on which visual and climatic selection is acting. Visual selection is imposed by predators choosing the more visually exposed individual. Climatic selection can act on the phenotype of the individual, as for example dark snails retain heat better than lighter coloured snails. This can affect the frequency at which certain phenotypes are found at particular altitudes.
We will be taking samples at a number of different locations at similar altitudes, which will each consist of an intermediate zone and two types of habitat on either side considered to be more isolated making migration from different types of habitats less likely. Selection and genetic drift will therefore have more impact than gene flow at these locations. We examine variations in allele frequency in terms of both banding and colouration between the different sites.
The intermediate zone is expected to show a higher rate of polymorphism than the habitats on either side, these areas are expected to show morphs with a phenotype favoured by visual and climatic selection.
A wide variety of shell colours and bandings in intermediate zones would support the theory that gene flow does in fact affect the allele frequencies, maintaining polymorphism in the population. In comparison, phenotypically uniform populations in habitats on either side and some distance from the intermediate zone would support the hypothesis that gene flow has smaller impact on allele frequencies in areas exhibiting more consistent habitat.
If however allele frequencies in samples from similar habitats differ markedly, this might suggest that genetic drift has had an impact on the population or that the selection pressures differ between locations. This should be taken into account when reviewing results. This experiment assumes that selection works against maintaining polymorphism. A confounding factor could be the apostatic selection presented by the predatory birds.
Two degrees of freedom and 0.05 critical value (5.99)
Location 1
Colour data:
Banding data:
No significant difference in morph frequencies for colour or banding. We see the expected trend in the grassy area with a higher frequency of yellow and 4-5 banded shells. The intermediate area showed a variety of polymorphism, slightly skewed towards yellow shells. In the shrubby area we expect a high ratio of pink/brown coloured shells, however we found more yellows. This area was relatively grassy, so selection would favour yellow, banded as they would be better camouflaged. Gene flow from the grassy area could be affecting the shrub and intermediate zone.
Location 2
Colour data:
Banding data:
Significant differences in frequency for both colour and banding, with a high number of yellow in both grassy and intermediate zones, with more variation in the intermediate zone than expected. The trend was shown in the shrub area, which was woodier than location one, pink/brown unbanded shells were most frequent. This obeys the expected trend; variation was found to be highest in the intermediate zone, suggesting that the ‘confined’ zones would be subject to selection favouring yellow in grassland and pink/brown in shrub area.
Location 3
Colour data:
Significant differences in morph frequency for both colour and banding as expected, the grassy area exhibited a high frequency of yellow, banded shells. The intermediate area showed a variety of polymorphism, slightly skewed towards yellow and banded which indicated gene flow from the grassland or selection favouring this phenotype. The shrub area showed the expected higher frequencies of pink/brown, unbanded.
Location 4
Colour data:
Banding data:
Chi square: 1.83 - null hypothesis rejected
No significant morph frequency for colour and banding. As expected, in grassland yellow, banded were most frequent, however this trend was not as pronounced as other locations. The intermediate area showed variety, though perhaps because the grassland and shrub locations either side had more yellow, pink/brown and banded this was reflected in our findings. The shrub area did not exhibit expected frequencies as there was a majority of banded shells but overall pink/brown shells were more frequent.
Discussion:
The discrepancies we found in certain locations could have arisen from sampling errors or bias which might have caused anomalous results. For example our sample size was too small with only 10 individuals in each. In location 3 the sample could have been larger, but in the remaining areas we struggled to find even 10 individuals however we aimed to keep the sample size consistent across the locations we sampled. The grassland and shrub areas that we sampled could have been more confined; in reality there was a more gradual change in habitat which could account for the data being skewed towards yellow and banded shells.
The fact that so few pink/brown shells were found could be due to them being harder to see. In the shrub area where these would be expected we were forced to search underneath vegetation and leaf litter, which may have caused us to overlook the brown shells. Yellow and pink were much easier to spot which would produce a bias. Location 1 might differ because this was our first attempt and from here on we were more aware of what we were looking for thus searching became more efficient.
Overall our results support our hypothesis that polymorphism is more pronounced in the intermediate zones which is likely to be due to the actions of gene flow from populations in confined areas. The confined populations are expected to exhibit less polymorphism as they are subject to both selection and genetic drift. Considering previous results (Jones et al 1977) it would be expected that in a shrub area you would find more brown unbanded snails whereas in a grassland area there would be more yellow banded snails. This is because predators are prone to select their prey on the basis of appearance so camouflage within a habitat will provide a selective advantage. However in location 1 the distribution of shell colours was skewed towards yellow which could be a result of genetic drift. Location 4 also differed from our expectations in that the shrub area did not seem to be as confined as it could have been. The results suggest that gene flow from outside populations could have skewed our results. The nature of these evolutionary processes means that the polymorphism seen in the snail populations can not be attributed to just one factor but a combination of effects will produce frequencies within a population.
Contributing members (all):
Henry Pyett
Richard Morrow
Laura Shakespeare
Kathrine Nielsen (group secretary)